Key C fits its lock. But I have thinned its stem and widened its hole so much,that I have almost ruined it. Two holes are visible on one side of the stem. However, the key is still resistent enough to stand further operations.

I have been filing the bit where the wards marks were visible. Now key C is finished and turns within the lock. Here is the picture. Quite intricated pattern, impossible to overcome by a wire.
The key turns but…
It does not open it. The key turns freely (clockwise or counterclockwise , several times) with no resistence and I cannot feel it lifting the spring or move the bolt.
I am wondering if this lock C was maybe not working at all, or if there is some trick – e.g. the locks must be operated in a certain order. But most probably, the blank had actually to be a bit longer and wider, so that the bit is not long enough to lift the spring.
Well, we will see.